1. Field of the Invention
The present invention generally relates to a structure of a recording medium. In particular, the present invention relates to an encoding structure of a holographic recording medium.
2. Description of Related Art
Along with the development of optical storage media, large capacity storage has been brought into reality due to the development of holographic techniques. The holographic storage technique uses two-dimensional images as a data access format, and is very different from ordinary optical storage operations and principles. The recording scheme for a holographic recording medium is based on a recording format composed of books and pages, in which a book comprises a plurality of pages, and each page is recorded with diffractive holographic images so as to record the desired data.
In a holographic recording medium system, in order to identify the data images accurately, at least one fixed area is usually specified in a holographic page, and a special pattern (referred as reference mark (RM) thereinafter) is placed into the fixed area for serving as references of positioning and correcting the holographic images. There are two methods for image positioning and correction. One of the methods is to move at least one specific element in the holographic recording medium system to allow the images to reach an optimal read state, and the other method is to directly calculate image data captured by an image sensor (for example, a CCD) through a digital image processing technique. At least one reference index is required for either of the foregoing two methods. Placing a reference mark RM in a holographic page is the most direct and effective way to obtain corresponding information.
Furthermore, in addition to general data to be recorded, data or information related to the system or the recording medium, such as book address, page address, capacity, or manufacturer information, etc., are also recorded in the holographic recording medium. Such data or information is expected to be correctly identified before the data images are in the optimal read state. Thus, each bit in such data or information is usually expressed with more pixels so that the data or information can be identified more easily than other general data.
A data recovery method is disclosed in U.S. Application No. 2005/0018263. As shown in FIG. 1, a two-dimensional data image 10 is projected on SLM and is stored in a holographic recording medium, and there are four special patterns A, B, C, and D in fixed regions of the holographic recording medium. An data image 12 recorded in the holographic recording medium can be captured by a camera by reading the holographic recording medium, and the four special patterns A, B, C, and D are similarly captured in the data image 12. Compared to the two-dimensional data image 10 projected on SLM, the data image 12 captured by the camera is usually enlarged, reduced, or even twisted due to the design of optical paths of the system and other optical interferences. If the positions of the special patterns A, B, C, and D in the data image 12 captured by the camera can be detected, the data image 12 can be calibrated into an image close to the two-dimensional data image 10 projected on SLM through an interpolation technique (for example, perspective transform or fractional-delay filter interpolation, etc.) based on the correspondence between the positions of the special patterns A, B, C, and D in the data image 12 and the known position information of the special patterns A, B, C, and D in the two-dimensional data image 10. However, the special patterns in this method are only used for calibrating images.
A holographic data storage format is disclosed in U.S. Application No. 2005/0250046. As shown in FIG. 2, the holographic page 20 is divided into a main data recording area 22 and an auxiliary recording area 24. The main data recording area 22 records general data, and the auxiliary recording area 24 records the related information of the holographic page, such as address information. The two areas may have different recording patterns. However, only a conceptual data storage format is provided in this patent to separate the recorded data and the related information thereof, and the data images in the auxiliary recording area cannot be used for calibrating images.
In foregoing two methods, the positioning and calibration of holographic images and the recording of additional information are designed separately. Thus, the system becomes complicated no matter what methods are used. Accordingly, how to simplify the structure of a recording medium and provide an encoding method that can calibrate images and hide information at the same time is one of the most important subjects in the field.